This invention relates to a proximity detector, and especially to a ferrous-gear-tooth Hall-transducer, or other magnetic-field-to-voltage transducer, capable of detecting the leading and trailing gear tooth edges of an adjacent rotating ferrous gear, or other magnetic articles, and more particularly relates to such a Hall proximity detector having a transducer-signal detector with a dual-mode hysteresis feature.
The term "magnetic article" as used herein applies to magnetized bodies, ferrous bodies and other bodies having a low magnetic reluctance that tend to alter the ambient magnetic field.
Conventional proximity detectors have consisted of a transducer, e.g. a Hall element, and a transducer-signal detector which produces a binary output (Vout) having transitions corresponding to predetermined points on the excursions in the transducer-voltage signal (V.sub.H).
The most common type of transducer-signal detector has been the transducer-signal mid-signal detector. A mid-signal detector is one that includes a comparator having hysteresis and includes other circuitry which biases the center of the comparator-hysteresis loop at a level corresponding to approximately the middle of the transducer signal.
Such simple mid-signal detectors are employed in the proximity detectors described in the U.S. Pat. No. 4,374,333 issued Feb. 15, 1983, U.S. Pat. No. 4,443,716 issued Apr. 17, 1987 and U.S. Pat. No 4,705,964 issued Nov. 10, 1987. In the mid-signal detector, the points in the positive and negative signal-excursions at which the binary output signal transitions occur are determined by the voltage thresholds of the Schmitt circuit hysteresis. In U.S. Pat. No. 5,650,719 issued Jul. 22, 1997 a mid-signal peak-to-peak-percentage-threshold detector is used and the thresholds are updated periodically to be a predetermined percentage of the peak to peak voltage of the transducer signal.
In the U.S. Pat. No. 5,442,283, issued Aug. 15, 1995, and assigned to the same assignee as is the present application, there is described a Hall-voltage proximity-detector capable of detecting the approach and retreat of an adjacent rotating gear tooth. This proximity-detector includes an integrated circuit including a Hall element and Hall-signal peak-referenced-threshold detector mounted to a pole of a magnet. This peak-referenced-threshold detector is a circuit for tracking a slope of a Hall voltage and briefly holding the ensuing peak Hall voltage until the Hall voltage falls away from the held peak by a predetermined voltage (threshold), at which time a transition is caused to occur in the binary output signal of the detector indicating the approach (or retreat) of a magnetic article.
The proximity detector described in U.S. Pat. No. 5,694,038 issued Dec. 2, 1997, entitled DETECTION OF MAGNETIC ARTICLES WITH AUTOMATIC GAIN CONTROL, also includes a Hall signal detector of the peak-referenced-threshold type, as does the proximity detector of the present invention.
Proximity detectors of either type in the prior art produce a high binary output voltage having positive and negative going binary transitions indicating approach and retreat of a passing article. A peak-referenced-threshold type signal detector, similar to that in the above-noted U.S. Pat. No. 5,442,283, is described in the U.S. Pat. No. 5,729,103 issued Mar. 17, 1998 that is entitled DETECTION OF MAGNETIC ARTICLES AT SPEEDS DOWN TO ZERO. All three of the fore-mentioned patents U.S. Pat. No. 5,650,719, U.S. Pat. No. 5,694,038 and U.S. Pat. No. 5,729,130 are assigned to the same assignee as is the present invention.
Prior art proximity detectors use Hall signal detectors of the mid-signal or peak-referenced-threshold types. These detectors have a fixed threshold voltage, and produce low to high (or high to low) binary transitions in the output signal indicating approach of a passing magnetic article.
In practice the closest passing distance, sometimes referred to as the air gap, does not remain constant during a given detection period, or during the life of the detector. And of course identical detectors are never installed so that the air gap is always the same from installation to installation. The amplitude of the signal voltage from the magnetic-field-to-voltage transducer used in proximity detectors is a strong inverse function of the air gap, and thus the range of air gap dimensions over which a prior art proximity detector can properly operate is severely limited.
Changes in the air gap, between the transducer and the passing articles, may be attributable to the instability of the path of the passing magnetic articles, e.g. degree of eccentricity of rotating gear teeth, or the degree to which the articles have been identically positioned on a passing conveyer belt.
But of greatest concern for detecting the passing of magnetic articles, is the mounting of proximity detectors adjacent to rotating gears at air gap spacings that vary from installation to installation. This leads to inaccuracies of article detection, which is especially distressing when used in critical applications such as in combustion engine ignition systems.
Detection accuracy, as used herein is meant to refer primarily to the degree of consistency in the correspondence of the timing of the signal detector output transition to the time when the approach (or retreat) distance between the Hall element and a passing article is one particular distance. Detection accuracy of course also includes the absence of false detection indications of passing article approaches or retreat.
It is an object of this invention to provide a proximity detector, that generates transitions in the detector binary output voltage which transitions accurately correspond to consistent points of approach and retreat of passing magnetic articles over a wide range of air gap dimensions.
It is another object of this invention to provide such a proximity detector for preventing a false detection indication due to switching on a trailing overshoot portion of an excursion in the waveform of the transducer voltage.
It is a further object of this invention to provide such a proximity detector that at start-up quickly switches from a high to a low detection threshold when the transducer voltage exceeds a predetermined reference voltage so that excepting for only the first one or two passing articles at start-up, the accuracy of detection will be improved during the following detection session, until detection is stopped and restarted again.